While still in the mother's womb, a baby doesn't need his or her lungs to supply oxygen because the baby receives oxygen via the mother's lungs and placenta. Since a baby's lungs do not provide oxygen, there is no need for energy to be expended pumping blood to the lungs. The ductus arteriosus is a blood vessel that is present in all babies while still in the womb that allows blood to bypass the pathway to the lungs; it allows blood to flow from the pulmonary artery to the aorta.
When the baby is born and the umbilical cord is cut, the lungs are now needed to supply oxygen. The lungs expand, their blood vessels relax to accept more flow and the ductus arteriosus usually closes within the first hours of life. On occasion, however, the ductus arteriosus does not close on its own. This is referred to as a patent ("open") ductus arteriosus. While this condition is much more often seen in premature babies, it may also appear in term infants.